Digital broadcast technology enables multiple broadcast programs (multiple logical channels) to be multiplexed and transmitted simultaneously on a single physical channel (frequency band). Specifically, stream data referred to as an elementary stream (ES) are generated for each broadcast program from the encoded streams (encoded video stream, encoded audio stream, etc.) of multiple broadcast programs. The transmitter can divide the stream data, multiplex the divided stream data, and transmit the multiplexed data on a single physical channel.
When a digital broadcast receiver performs a tuning process, it first sets its tuner to the particular frequency band on which the target broadcast program is carried, and receives the broadcast signal. Next, the digital broadcast receiver obtains information (referred to below as tuning information) necessary for tuning from the digital broadcast stream obtained by receiving the broadcast signal. Then, on the basis of the tuning information, the digital broadcast receiver separates the encoded streams (encoded video stream and encoded audio stream) of the target broadcast program, which are multiplexed in the digital broadcast stream, and decodes the separated encoded streams, thereby reproducing a video picture and sound.
For example, Japanese terrestrial digital broadcasting uses the transport stream format specified by the MPEG-2 system (Moving Picture Experts Group phase 2 system). According to the MPEG-2 system, an ES (Elementary Stream) generated from an encoded stream is divided into data blocks of a suitable size and converted to packet stream data referred to as a PES (Packetized Elementary Stream). Further TS packets (Transport Stream packets), each having a fixed length, are generated from the PES. The transmitter modulates and transmits a transport stream (TS) generated by multiplexing of these TS packets.
The digital broadcast receiver receives the broadcast signal and obtains the digital broadcast stream, i.e., transport stream (TS, below). The digital broadcast receiver refers to a packet identifier (PID) included in the header of each TS packet in the TS and filters the TS to obtain only TS packets relating to the target broadcast program. The digital broadcast receiver can then perform audio output and video output by assembling the filtered TS packets into audio and video ESes and decoding each ES. The Japanese terrestrial digital broadcast standard is disclosed in ARIB TR-B14 Ver. 3.9 (Non-patent Document 1), ARIB STD-B32 Ver. 2.2 (Non-patent Document 2), and other ARIB-defined specifications.
The ARIB standard refers to the broadcast programs corresponding to each encoded stream as a service. The broadcast programs, or service, are also referred to as an organized channel, which means a sequence of scheduled broadcast programs (events) organized by a broadcast provider. In tuning to a specific service, program identification information called PSI (program specific information), which is defined by the MPEG-2 system standard, is used as tuning information. The PSI may include four tables: a PAT (program association table), PMT (Program Map Table), NIT (Network Information Table), and CAT (Conditional Access Table). The PAT specifies the PID of the TS packet carrying the PMT relating to the service; the PID of the TS packet carrying the PAT is set to a value of zero. The PAT stores, for all services included in the TS, the service ID (service_id), which is the identification information of each service, and the PID of the TS packet carrying the PMT. The PMT, which is present for each service, is information specifying the PIDs of the TS packets carrying the encoded streams constituting the service.
When a digital broadcast receiver conforming to the ARIB standard performs a tuning process, it first sets its tuner to the particular frequency band on which the target service is being broadcast, and receives the broadcast signal (the frequency tuning process). Next, the digital broadcast receiver refers to the PIDs of the TS packets and receives the PATs, from which it obtains the service IDs. On the basis of the obtained service IDs, the digital broadcast receiver selectively receives a PAT corresponding to the target service and, from the PAT, obtains the PID of the TS packet carrying the PMT corresponding to the target service. The digital broadcast receiver then selectively receives the PMT from the obtained PID, and on the basis of the PMT, it filters the TS packets to obtain those including the encoded audio and video streams and PCR (Program Clock Reference). Sound and picture output can be performed by assembling audio and video ESes from the filtered TS packets and decoding these ESes. Note that a PCR is time stamp information with which the decoder replicates the reference time used when the audio data and video data were encoded.
As described above, the time required for the tuning process is at least the sum of the time required for the frequency tuning process, the time required for selective reception of a PAT (PAT reception waiting process), the time required for selective reception of a PMT (PMT reception waiting time), and the time required for the ES decoding processes. Therefore, as methods of speeding up the tuning process, several methods (referred to below as fast tuning methods) for performing the tuning process by referring to a PAT, PMT, and other tuning information that are obtained beforehand in some way have been proposed. When a fast tuning method is used, the PIDs of the encoded streams and PCRs of each service are stored in advance and the TS packets are filtered on the basis of these PIDs, whereby the decoding process can start shortly after the frequency tuning process.
When the tuning process is performed by use of a fast tuning method, if the content of the previously obtained tuning information has been changed, there is a possibility that the tuning process will be performed incorrectly. For example, the PIDs of an encoded stream described in the PMT might be altered during a broadcast. Therefore, if the TS packet filtering and decoding processes are carried out on the basis of the previously stored PIDs of the encoded stream, a situation might occur in which picture or sound reproduction fails, or the picture of one service and the sound of another service are reproduced simultaneously. In order to avoid such situations, in the tuning process it is necessary to perform a process of checking for the presence of alterations in the tuning information, and when an alteration of the tuning information is recognized, the tuning process must be performed by use of newly received tuning information. One fast tuning method of this type is disclosed in Japanese Patent No. 3549442 (Patent Document 1).
In the fast tuning method in Patent Document 1, the ECM (Entitlement Control Message) employed in scrambled broadcasting is used as tuning information in addition to the PAT and PMT, and the PID of the PMT and the PID of the ECM are recorded in advance for each service. The TS packets carrying the PAT, PMT, and ECM are filtered concurrently, and whether the PIDs of the received PAT, PMT, and ECM match the recorded PIDs of the PAT, PMT, and ECM is determined. The fast tuning method in Patent Document 1 speeds up the tuning process by concurrently receiving multiple items of tuning information, which were received sequentially in the conventional tuning process.